1. Field of the Invention
The present invention relates generally to a semiconductor device, and more particularly to an improved semiconductor device having an interconnection (wiring) layer and being capable of preventing a short-circuit between the wiring layer and another wiring layer with different potential or between the wiring layer and a semiconductor substrate.
2. Description of the Background Art
FIG. 3 is a sectional view of a semiconductor device such as a semiconductor memory device having a conventional structure of multilayer interconnection.
Referring to FIG. 3, an interlayer insulating film 5a is formed on a semiconductor substrate 1. A first conducting layer 2 is formed on interlayer insulating film 5a. An interlayer insulating film 5b is formed on interlayer insulating film 5a such that it covers first conducting layer 2. A second conducting layer 3 is formed on interlayer insulating film 5b. An interlayer insulating film 5c is formed on interlayer insulating film 5b such that it covers second conducting layer 3. A contact hole 6a is formed in interlayer insulating films 5a, 5b, and 5c in order to expose the surface of semiconductor substrate 1. A contact hole 6b is formed in interlayer insulating film 5c in order to expose the surface of second conducting layer 3. Contact hole 6b penetrates second conducting layer 3, interlayer insulating film 5b, and interlayer insulating film 5a, reaching the surface of semiconductor substrate 1. A contact hole 6c is formed in interlayer insulating films 5b and 5c in order to expose the surface of first conducting layer 2. A third conducting layer 4a is formed on interlayer insulating film 5c such that it is connected to the surface of semiconductor substrate 1 through contact hole 6a. A third conducting layer 4b is formed on interlayer insulating film 5c such that it is connected to second conducting layer 3 through contact hole 6b. Third conducting layer 4b is connected to first conducting layer 2 through contact hole 6c.
Referring to FIG. 3, in a conventional semiconductor device, conducting layers are connected to each other through contact holes formed in an interlayer insulating film to connect two conducting layers formed across different layers.
As shown in FIG. 3, however, the total thickness of interlayer insulating films 5a, 5b and 5c formed between third conducting layer 4a and semiconductor substrate 1, the thickness of interlayer insulating film 5c formed between third conducting layer 4b and second conducting layer 3, and the thickness of interlayer insulating films 5b and 5c formed between third conducting layer 4b and first conducting layer 2 are different. Therefore, when contact holes 6a, 6b and 6c are formed simultaneously, contact hole 6b penetrates second conducting layer 3 and reaches the surface of semiconductor substrate 1, by the etching to form contact hole 6a reaching the surface of semiconductor substrate 1.
Such a penetration occurs when etching selectivity of second conducting layer 3 to interlayer insulating film 5c is small. In other words, penetration of second conducting layer 3 occurs when the difference between the etching rate of second conducting layer 3 and the etching rate of interlayer insulating film 5a, 5b, 5c is small. For example, the etching selectivity of interlayer insulating film 5c to materials such as polysilicon and doped polysilicon is small.
Thus, in the conventional device, when second conducting layer 3 is formed of a material of small etching selectivity to interlayer insulating film 5c, contact hole 6b penetrates second conducting layer 3. As a result, a short circuit is formed between one wiring layer and another wiring layer with different potential or between a wiring layer and a semiconductor substrate.